BMI calculator

How to use the BMI calculator

Start by choosing Adult (18+) or Child (2–17). This is required: the calculator shows the bare BMI number until you choose, because an adult category given to a child would be wrong and could be alarming or falsely reassuring. For a child you then choose sex at birth and enter the age, either in years and months or as a date of birth with the date the child was measured.

Enter height and weight in whichever units you think in. Height can be in centimetres or feet and inches; weight in kilograms, stones and pounds, or pounds alone. You can mix them, and when you switch a unit the number you've already typed is converted rather than lost. The calculator also checks that the numbers make sense for the unit: typing 1.75 in the centimetres box offers Use 175 cm, 5.9 in centimetres offers 5 ft 9 in, and 158 in the stones box offers 158 lb. No category is shown until the units look right.

For adults the optional fields add more: your family background switches to the lower thresholds NICE recommends where relevant, sex and waist add the NHS waist check, and age, sex and activity level give a daily calorie estimate. The Weight for a BMI of box works backwards from any BMI. Everything is calculated in your browser and nothing is saved except your preferred units.

BMI ranges for adults

Body mass index is your weight in kilograms divided by the square of your height in metres. Someone 1.75 m tall weighing 70 kg has a BMI of 70 ÷ 1.75² = 22.9. The NHS uses these adult ranges, which follow the World Health Organization's:

NICE guidance recommends the lower thresholds for adults with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, because the risk of type 2 diabetes and other conditions rises at a lower BMI in these groups. Choose the matching background in the calculator to apply them. The category is decided from your BMI rounded to one decimal place — the figure you see — so a BMI shown as 25.0 is always in the overweight range, never the healthy one. The healthy weight range uses the same rule: it runs from the lightest to the heaviest weight, in 0.1 kg or 1 lb steps, whose rounded BMI is healthy, so at 175 cm it reads 56.6–76.4 kg and 76.4 kg is shown as healthy.

BMI for children and teenagers: UK90 centiles

Children's body fat changes a great deal as they grow, so a BMI of 18 can be low for one child and high for another. Instead of fixed ranges, a child's BMI is compared with children of the same age and sex using the UK 1990 BMI reference (Cole, Freeman and Preece, 1995), the data behind the RCPCH growth charts and the NHS healthy weight calculator for children. The calculator carries the full monthly UK90 table for boys and girls from 2 to 18 and uses the LMS method: for the child's exact age it looks up three numbers — L (skewness), M (median) and S (spread) — and works out z = ((BMI ÷ M)^L − 1) ÷ (L × S). The z-score converts to a centile: the 50th centile is the median, and the 91st means the BMI is higher than 91 in 100 children of that age and sex in the reference.

As an example, a 12-year-old girl who is 150 cm and 50 kg has a BMI of 22.2, which sits on the 92nd centile — the overweight band — although the same BMI would be healthy for an adult. The calculator also shows the healthy weight band for the child's current height and age, but children grow quickly and one reading matters less than the trend over time. BMI isn't used under 2; babies and toddlers are plotted on the UK-WHO charts in the red book. Between 2 and 4 the NHS charts use WHO data, so a health visitor's centile may differ slightly. To work out an exact age, the age calculator helps.

Waist-to-height ratio, BMI Prime and Ponderal index

• Waist-to-height ratio is your waist divided by your height, in the same units. It reflects fat carried around the middle, which is more closely linked to heart disease and diabetes than weight alone. NICE describes 0.4 to 0.49 as healthy central adiposity, 0.5 to 0.59 as increased and 0.6 or more as high. The simple rule is to keep your waist to less than half your height.
• BMI Prime is your BMI divided by 25, the upper limit of the standard healthy range. A value of 1.00 sits exactly on that line; 1.20 means 20% above it. It makes comparisons between populations easier but tells you nothing BMI doesn't.
• Ponderal index divides weight by height cubed instead of squared. Because it scales better with height, it is less likely to overstate things for very tall people or understate them for very short ones. Typical adult values sit around 11 to 15 kg/m³.

For the most useful picture, look at BMI and waist-to-height ratio together. If you want to convert a measurement taken in inches or pounds before entering it, the unit converter handles stones, pounds and inches exactly.

What BMI can and can't tell you

BMI is a screening measure for populations, not a diagnosis for individuals. It doesn't know whether weight is muscle or fat, so athletes and people who do a lot of strength training often read as overweight when they aren't carrying excess fat. It can also look reassuring in older adults who have lost muscle, and it doesn't apply during pregnancy. It says nothing about where fat is stored, which is why the waist measurement matters: the NHS suggests men aim for a waist below 94 cm (90 cm for South Asian men) and women below 80 cm.

The calorie estimate uses the Mifflin-St Jeor equation multiplied by an activity factor. It is a starting point accurate to roughly 10% for most people, not a prescription. If your result surprises you or you're worried about your weight, a GP or practice nurse can take proper measurements and look at your wider health, and the NHS offers a free 12-week weight-loss plan.

Frequently asked questions